It relates more particularly to cases in which the couplers in question comprise:
two tubular rigid strength members or frames surrounding each other, and preferably of revolution and coaxial at least under load, which strength members are able to be solidly connected to the two rigid elements to be joined respectively,
a cross-brace body made of elastomer connecting together the two strength members and forming, at least partially, with the latter two fluid-tight chambers which are deformable in opposite directions senses when one of the two strength members moves with respect to the other in a diametral direction E,
a restricted passage causing the two chambers to be permanently in communication with each other,
a liquid mass filling the two chambers and the restricted passage,
a rigid movable member delimiting in part at least one of the two chambers and mounted in such a manner as to be able to move with a limited amplitude.
As is known with such a coupler:
when oscillations of relatively large amplitude (namely greater than 0.5 mm) and of relatively low frequency (namely of the order of 5 to 15 Hz), such as those due to the "chopping" generated on the vehicle by the jolts from the road, are applied in the direction E to one of the two strength members, the liquid of one of the two chambers is displaced into the other one, and vice-versa, through the restricted passage, setting into resonance the liquid mass thus displaced when the frequency of said oscillations reaches a predetermined value which is a function of the ratio between the axial length and the cross-section of the restricted passage, this setting into resonance ensuring an excellent damping of the oscillations in question,
when vibrations of relatively small amplitude (less than 0.5 mm) and of relatively high frequency (generally between 20 and 200 Hz), such as those due to the operation of a vehicle engine, are applied to one of the two strength members the movable member is the seat of vibrations capable of attenuating or filtering the transmission of the vibrations concerned.
In known embodiments of the couplers of the type in question, the movable member is generally free to move between its two stops and it is therefore the vibrations to which it is naturally subjected as a function of the vibrations to be damped which are exploited in order to achieve the desired attenuation.
In order to reinforce this attenuating or filtering effect, it has already been contemplated to suppress the "natural" character of the useful and exploitable vibrations of the movable member by modifying them in an artificial and forced manner, a method sometimes referred to under the name of "active attenuation".
For this purpose it has been proposed that the movable member is constituted by, at least partially, a ferromagnetic element, to apply "counter-vibrations" to this element with the help of electrical coils that is to say alternating forces of the same frequency as the vibrations to be reduced and which are oriented in the same direction as these vibrations and of reverse sense and to take a certain number of measures in order to permit stringent control of the controlled displacements of the flap and especially to prevent the latter from coming into contact with its stops when one of the two component strength members of the coupler is subjected to oscillations of low frequency and large amplitude.
These measures are in particular the following:
the movable member is guided by at least one deformable ring so as to move only in translation in the direction E of the vibrations to which it is subjected,
spaces or air-gaps between the movable member and the coils are separated from the damping liquid by at least one fluid-tight annular seal,
and provision is made for an electronic system acting as servo-control the electric excitation of the coils to the movements of the movable member itself such that the excitation due to this single system has the effect of reducing, which includes cancelling, these movements, said system comprising for this purpose a sensor of said movements arranged in the immediate proximity of said movable member (French Patent Application No. 90 01075, corresponding to U.S. application Ser. No. 07/648,526, filed Jan. 30, 1991, now U.S. Pat. No. 5,116,029, issued May 26, 1992).